Abstract
Background: Ectopic thyroid tissue (ETT) can occur anywhere along its embryological descent, most commonly at the tongue base and neck. While superior mediastinal occurrence is extremely rare, such cases are usually asymptomatic and detected incidentally. Differentiating ETT from other masses in the mediastinum can be challenging using conventional imaging, and thyroid scintigraphy provides valuable functional information for accurate diagnosis and management.
Case Presentations: In our first case, a 24-year-old female presented with a complaint of swelling in the neck region. The thyroid scan with pertechnetate injection was performed, which showed foci of abnormal increased uptake inferior to the normal thyroid gland, corresponding to soft tissue nodules in the superior mediastinum on correlative single-photon emission computed tomography/computed tomography (SPECT/CT). In our second case, a 32-year-old female presented with a complaint of neck swelling. A focus of abnormal increased uptake was noted inferior to the thyroid gland on thyroid scan. Correlative SPECT/CT revealed the abnormal uptake to be a soft tissue nodule in the superior mediastinum with a similar Hounsfield unit to the orthotopic thyroid gland, suggestive of ETT.
Conclusion: ETT in the superior mediastinum, in the presence of a normally located thyroid gland and normal thyroid function tests, is a very rare finding. The combined use of thyroid scintigraphy and SPECT/CT in such cases allows precise anatomical and functional characterization of the tissue, thereby guiding appropriate management and potentially avoiding unnecessary invasive procedures.

Key words: Ectopic thyroid tissue, superior mediastinum, thyroid scintigraphy, single photon emission tomography/computed tomography, case report.